Field of the Invention
The present disclosure relates to an image capturing apparatus and a control method therefor, and particularly relates to an image capturing apparatus capable of shooting (high dynamic range shooting) for generating a high dynamic range image and a control method therefor.
Description of the Related Art
A technique called high dynamic range (HDR) composition for compositing a plurality of images captured with different exposure amounts (underexposure and overexposure) to expand a dynamic range of an image has been known. With this technique, by compositing standard exposure portions in the images, blown-out highlights and blocked-up shadows can be suppressed, and an image having a favorable tone from a dark portion to a bright portion can be obtained.
The HDR composition (HDR moving image capturing) is able to be applied to moving image capturing when image capturing is repeated with different exposures in an alternating manner, for example, in order to acquire images with different exposures (underexposure and overexposure) for each frame of a moving image. However, when the HDR moving image capturing is performed under exposure conditions of underexposure and overexposure relative to standard exposure fixed, there is no difficulty in a case of a scene having a great brightness difference, but in a case of a scene with a small brightness difference, the dynamic range is so wide that contrast of an image obtained by compositing an underexposure image and an overexposure image is reduced. Therefore, a bright portion and a dark portion of the scene are detected and a detection result thereof is fed back to the exposure conditions of the underexposure image and the overexposure image for performing automatic control. In this case, since values of the exposure conditions of the underexposure image and the overexposure image are not always symmetrical with respect to the standard exposure, an exposure difference between the standard exposure and the underexposure and an exposure difference between the standard exposure and the overexposure are not the same in some cases.
When HDR moving image capturing is performed by composting three (standard-exposure, underexposure, and overexposure) images, hardware allowing high-speed reading, by which image capturing is able to be performed at 90 frames per second (90 fps) is required to generate a moving image having 30 frames per second (30 fps). Thus, it is desired to realize the HDR moving image capturing with two (underexposure and overexposure) images.
Under such circumstances, Japanese Patent No. 4402710 describes a method for performing pre-photographing before actual photographing of a still image in HDR composition, obtaining a histogram of luminance from the image obtained by the pre-photographing, and determining standard exposure to take a balance between blown-out highlights and blocked-up shadows. Japanese Patent No. 5257487 describes that a histogram of luminance of a composite image obtained by compositing an underexposure image and an overexposure image is checked, and when the histogram is inappropriate, the underexposure or the overexposure is corrected, and that a starting time of exposure correction is considered by timer processing in order to suppress a variation.